The fingerprint sensing industry uses several different conventional technologies to capture images of an individual's fingerprints. Two prominent technologies are optical-based sensors and capacitance-based sensors. In a typical optical sensor, a light source, lenses and a prism are used to image the ridges and valleys on a fingerprint, based on differences in the reflected light from the features. Conventional capacitance sensors include two-dimensional array of capacitors defined on a silicon chip, and fabricated by semiconductor CMOS processing. The individual sensors on the chip form one plate of the parallel plate capacitor, while the finger itself, when placed on the array, acts as the second plate for the various localized sensors. Upon contact with the array of sensors, the individual distance from each sensor to the corresponding point on the skin above the sensor is measured using capacitive techniques. The difference in distance to skin at the ridges and valleys of a fingerprint identifies the fingerprint.
Capacitive and optical sensors can be sensitive to oils or grease on the finger and to the presence or absence of moisture on the finger. In addition, the ambient temperature can affect these sensors at the time of sensing. Under hot or cold conditions, capacitive sensors can provide erroneous readings. Finally, most sensors have abrasion resistant coatings. The thickness of the protective coating can affect the measurements. The combined effect of these variables can result in distorted fingerprint images. Finally, in the case of silicon chip based fingerprint sensors, the placement of the finger directly onto the silicon increases the risk of electrostatic discharge and damage to the sensor.
The above-referenced U.S. patent application Ser. Nos. 09/571,765 and 10/038,505 describe systems and methods for performing texture (e.g. fingerprint) measurements using switch arrays. A fingerprint sensor for performing such measurements can include tens of thousands of miniature switches arranged in an x-y array. Each switch includes a lower electrode, and an upper electrode disposed over the lower electrode. Depending on the force applied on the switch, the upper electrode can be separated from the lower electrode, or can establish electrical contact with the lower electrode. The state (open or closed) of each switch indicates whether a fingerprint ridge or valley is positioned above that switch. A map showing the distribution of switch states over the sensor area can thus be used to identify a fingerprint positioned over the sensor.
In a representative implementation of such a fingerprint sensor, each switch extends over a few tens of μm along the x- and y-directions, and is capable of changing state upon the application of a load corresponding to a few mg of mass. Such a switch can include upper and lower electrodes formed by thin films having thicknesses on the order of tenths of micron to a few microns. The performance of such a switch can depend to a significant extent on the properties of the thin films defining the electrodes. Systems and methods for systematically evaluating such thin films could be of great benefit to the design of more accurate and reliable switches for texture sensing and other applications.